Nitrate Management in Beef Cattle
Nitrate poisoning is on the increase in North Carolina and beef producers who maintain moderate to high levels of nitrogen fertility should become familiar with how to prevent nitrate from accumulating in forage plants. If a producer must utilize high nitrate forages, a number of management considerations may be employed to limit the risk of economically devastating losses associated with acute nitrate poisoning. Hay producers should also develop an understanding of nitrate management to avoid liability for toxicity resulting from the sale of high nitrate hay.
Nitrate accumulation in forage plants
Nitrate levels can increase to toxic levels in forages any time the nitrogen supply in the soil exceeds the nitrogen needs of the plant. Plants absorb nitrogen from the soil in the form of nitrate that is later converted to protein. When nitrate is available to the plant but protein synthesis is slow, the nitrate will accumulate, especially in the lower part of the plant, until the rate of protein synthesis increases.
Nitrate accumulation in plant tissue is usually observed during times of drought, during long periods of cloudy or cool weather, or following herbicide applications. Anything that slows down the rate of plant growth can lead to increased nitrate levels in well-fertilized plants.
Some plants are "nitrate accumulators" which will often accumulate toxic levels of nitrate during drought conditions even though nitrogen fertility levels are moderate. These include summer annuals (millet and sorghum x sudangrass hybrids), johnsongrass, and annual weeds such as pigweed and lambsquarters. Other forage plants like bermudagrass, fescue, orchardgrass, small grains and ryegrass generally only accumulate nitrate under conditions of very high nitrogen fertilization (Table 1).
In the past, most concern for nitrate poisoning was focused on known nitrate accumulating plants. However, in recent years nitrate problems are becoming common in bermudagrass and fescue forages due to large amounts of animal wastes (and in some cases commercial fertilizers) being applied to pasture and hayland.
Nitrate toxicity in cattle
Nitrate itself is not toxic to the animal. In an unadapted animal, however, nitrate is converted to nitrite in the rumen which is then absorbed into the blood stream. Nitrite interacts with oxygen- carrying hemoglobin found in red blood cells, and results in the formation of methemoglobin which reduces the ability of the blood to transport oxygen. Blood high in methemoglobin is chocolate brown in color and this is sometimes observed in cattle suffering from acute nitrate toxicity.
The most common sign of acute nitrate poisoning is sudden death of one or more animals that appeared healthy the previous time they were observed. Early signs of acute nitrate toxicity will be seen within 6-8 hours after ingestion of a large amount of nitrate. Collapse, convulsions and death follow usually within 30 minutes of the onset of symptoms. Early signs of acute nitrate toxicity include labored breathing, frothing at the mouth, frequent urination, diarrhea, staggering, and a brown coloration of the mucous membranes. Less noticeable signs may be a poor breeding rate (due to abortions) and in some cases reduced gain. Producers should implement a forage testing program and employ other management strategies to eliminate cattle deaths and minimize the less noticeable problems.
The most common situation where significant cattle deaths occur is when a producer places hungry cattle that are not adapted to nitrate on high nitrate forages. Every fall, reports of cattle deaths come at the start of the winter feeding period. In this scenario, cattle have been allowed to graze the available pasture until there is little or no forage remaining and the animals are hungry. They are then fed round bales, and will eat a large amount rapidly from one section of the bale. If the hay is high in nitrate, and especially if there is a portion of the bale that is exceptionally high, then death is likely. Usually a producer has several cuttings of hay on hand and knowing nitrate levels allows the choice of using the lowest nitrate level at the start of the feeding period.
Another scenario where deaths commonly occur is when cattle are turned into summer annuals shortly after the end of a drought. When the plants receive sufficient moisture they can take up nitrate very rapidly, but growth of the plant, and therefore protein synthesis will be slower to resume. At this time, a producer wishes to get back to grazing as soon as possible and turns hungry cattle out when nitrate levels are at their highest. Allowing 1-2 weeks of forage growth before grazing in this scenario will minimize problems. Providing plenty of low nitrate hay for the animals to fill up on prior to allowing them to resume grazing on drought affected forage is another strategy for reducing the risk of toxicity.
Treatment of Nitrate Toxicity
The first step in treating a suspected outbreak of nitrate poisoning is to immediately remove potentially toxic forage to prevent additional cattle from being poisoned. Your veterinarian should be contacted immediately for diagnosis and assistance in handling cases of suspected nitrate toxicity.
Methylene blue has been used as an antidote for nitrate toxicity. When administered early, intravenous methylene blue has sometimes been an effective therapy. Methylene blue reduces methemoglobin (caused by nitrite) to hemoglobin, thus restoring the oxygen carrying capacity of the blood. Repeat treatments are usually necessary. Methylene blue is very irritating to tissues. Severe injury and necrosis to surrounding tissues may occur if the material is not injected intravenously.
The Animal Medicinal Drug Use Clarification Act (AMDUCA) permits veterinarians to use or prescribe an approved animal or human (if no approved animal drug exists) drug in an extra-label manner under certain conditions, especially if an animal may die if not treated. The veterinarian must have an established veterinarian-client-patient relationship in order to recommend the extra-label use of the drug.
However, since methylene blue is an unapproved new animal drug and is therefore not approved by the FDA for use in food-producing animals, it does not fit into the above category. The FDA does not have a specific written policy on the use of methylene blue in food producing animals. Additionally, human health concerns may be associated with its use in food animals.
The current FDA stance on the use of methylene blue in food producing animals seems to preclude this treatment as an acceptable option in the treatment of nitrate toxicity in food animals. Due to the rapid onset of symptoms, often with sudden death before treatment can be administered, and the lack of an antidote acceptable under AMDUCA, prevention is the key to managing nitrates in cattle production systems.
Preventing nitrate accumulation in forages
Nitrogen should be applied to a crop at agronomic rates. This means the amount of nitrogen added to a crop should not exceed its potential nitrogen uptake. In dry years, the crop will need less nitrogen than during wet years simply due to lower yield. Most non-leguminous forage crops can use no more than 40 to 50 lbs. of nitrogen per ton of yield in mechanically harvested systems. In grazing systems, much of the nitrogen consumed is returned to the soil in manure and urine, which means supplemental nitrogen needs may be as low as 20 to 25 lb. of nitrogen per ton of forage produced.
If nitrate levels are consistently high (above 0.25%) in non-accumulator plants such as bermudagrass, it indicates that there is too much nitrogen being applied to the soil, and annual nitrogen application rates should be reduced. Not only does the elevated nitrate pose a threat to the health of cattle, it also indicates that soil nitrate is high, resulting in potential for leaching of excessive nitrogen to shallow ground water, some of which could eventually enter surface waters. High nitrate levels in non-accumulator species may be the result of using forage crops as receivers for waste nitrogen where application rates are likely to be very high, in contrast to the use of commercial nitrogen sources where a high cost of nitrogen will result in more moderate application rates.
When applying nitrogen or harvesting forage, the producer should consider environmental conditions such as drought, cloudy weather, time of day, etc. Nitrogen fertilizer should not be applied during very dry weather, especially if there has been nitrogen applied earlier in the season. Wait several weeks after soil moisture is adequate before cutting forage for hay following a drought. Ensiling the crop, rather than harvesting as hay, will reduce nitrate by about half; therefore ensiling is a good idea if the crop is suspected to be high in nitrate.
Producers should understand the potential for nitrate accumulation in various plants (Table 1). Extreme caution should be employed when grazing fields that are infested with weeds that are known nitrate accumulators, or when grazing summer annuals during and shortly following drought stress. Producers with primarily non-accumulator species have little to fear from nitrate toxicity as long as nitrogen fertilization is not excessive.
Table 1. Accumulator and Non-accumulator plants
Feeding Management of High Nitrate Forages
The first step in managing high-nitrate forages is having a good testing program, and the key to a good testing program is obtaining a representative sample. At least 20%, or a minimum of 20 bales, should be sampled for each cutting of hay whether they are square or round bales. A "cutting" should represent hay that came from the same field, cut at the same time, and with similar management practices. The samples should be obtained with a core sampler rather than by pulling hay from a bale, which may result in misleading results due to uneven distribution of high nitrate forage within or between bales. If the analysis shows that nitrate is high in a given cutting of hay, the producer may desire to sample bales individually. Hay from different areas of a field can differ in nitrate level, especially if the fertilization rate is not uniform.
When obtaining pasture samples, the producer should walk in a random zig-zag pattern over the whole field and hand pluck a small area down to grazing height from about 20 locations. This process is similar to obtaining a good soil sample.
In a poisoning case that occurred in North Carolina in the winter of 1993-94, cows were fed round bales of a sorghum-sudangrass hybrid. The previous summer had several short dry spells, and the producer cut the forage several times and stacked it around the edge of the field with no knowledge of potential for nitrate toxicity. When feeding began, the producer offered the cows several bales of hay and, overnight, several animals died. Nitrate poisoning was diagnosed, and testing of the various forages available revealed that the sorghum x sudangrass had an average of 1.78% nitrate ion. Because the forage came from several different cuttings, bales were sampled individually, which showed that there was a great deal of variation from bale to bale. About half the bales were near 2.9% nitrate (which is typical of levels found where death loss is high) and these bales had to be destroyed. The other bales ranged from .25 to 1.39%, and these bales were fed to the cows. In this situation, individual bale sampling was required to safely feed the hay.
Nitrate Testing Services
Most state labs and commercial labs will analyze forages for nitrate. In North Carolina, the NCDA forage testing lab will, upon request, analyze a forage for nitrate, or add nitrate to standard forage quality analysis at no extra charge. The North Carolina lab will also notify producers immediately when they find high nitrate levels. Commercial labs usually add a small charge when nitrate is requested.
Understanding lab results. Labs commonly vary in the way they report nitrate levels. The two most common approaches are to report nitrate ion levels (percentage nitrate) or to report nitrate nitrogen in parts per million (ppm). Table 2 shows management considerations for high nitrate forage expressed either as percentage nitrate ion or ppm nitrate-nitrogen. Some labs also report nitrate ion in ppm, or nitrate-nitrogen as a percentage, so it is important to understand the conversion factor. To convert percentage to ppm, multiply by 10,000, and to convert from ppm to percentage divide by 10,000. For example if the result is reported as 0.35% nitrate-nitrogen, convert to ppm as follows: 0.35 x 10,000 = 3500 ppm nitrate N. If the result is reported as 8000 ppm nitrate ion, convert to percentage nitrate ion as follows: 8000/10000 = 0.8% nitrate ion.
Table 2. Management considerations for use in feeding forages with various levels of nitrate.
Feed high nitrate hay to animals in low risk groups. If a producer has hay with a range of nitrate levels the feeding guidelines in table 2 should be followed. As a general rule, within the guidelines of Table 2, hay with the highest levels of nitrate should be fed to animals in the lowest-risk category which would include open cows (following calving but before the breeding season) or growing calves (stockers or developing replacement heifers before breeding). This means a producer would need to refrain from feeding the higher nitrate hay until the end of the calving season and then finish feeding it before the breeding season begins. This is only possible on farms where there is a tightly controlled breeding season.
Blending forages. Blending of high nitrate forages with low nitrate feeds is a strategy to use whenever possible. This is especially helpful when using total mixed rations where intake of a given animal can be controlled. Blending may not be of much help when feeding hay free-choice because if offered one high-nitrate round bale and one low-nitrate bale, an individual cow may consume only the high-nitrate forage. To blend round bales, roll out one at a time so that all animals get some high- and some low-nitrate hay.
Utilize animal adaptation. Animal adaptation is a key management strategy for using high nitrate forages. Losses from nitrate toxicity will be much greater in animals not adapted to nitrate. Research has shown that growing, non-pregnant cattle can handle up to 5% nitrate if the level in their diet is increased gradually over several months. The bacteria in the rumen capable of degrading nitrite to ammonia for bacterial protein synthesis will increase in numbers when nitrate is available to them. If a producer analyzes forages for nitrate, gradual adaptation can be used as a strategy to minimize risk. Adapted animals can safely be fed higher levels as shown in Table 2. To adapt the cattle, start by feeding the lowest-nitrate hay and then work up to the highest. Even with adaptation, it is risky to feed hay free-choice with higher than 1.5% nitrate ion.
High nitrate pastures. Potential for nitrate toxicity is lower in grazed forage than in hay with the same level of nitrate due to several factors. Animals at pasture eat more gradually than those on hay, and the nitrate is released more slowly from fresh than from dried forage. Grazing animals also tend to be more adapted (usually the nitrate level changes gradually over the grazing period), and when cattle are grazing they tend to be selective and don't graze the entire pasture close to the ground (where nitrate is highest), while with hay cutting the forage is removed in a uniform manner.
Nitrate is high in stubble. As indicated above, nitrate level varies with location in the plant. Nitrate tends to accumulate in the lower stem, so cutting hay very short, or overgrazing so that cattle have to eat the lower stem bases can cause more intake of nitrate. This is a often a problem in millet or sorghum x sudangrass hybrids where leaving at least 6 inches of stubble is recommended, but cattle will eat it to the ground if they are hungry.
In research done in Sampson County, NC with bermudagrass irrigated with swine lagoon effluent, it was observed that under hay cutting conditions, the lower 3" of the forage contained 1.3% nitrate ion, while the portion above 3" contained only 0.7% nitrate ion.
Nitrate in drinking water. If it is known that cattle will be drinking high-nitrate water, this should be a consideration as well as forage nitrate levels. Water has to be quite high for it to add significantly to total nitrate intake. Nitrate in water is expressed as ppm nitrate-nitrogen. Assuming water and forage intake are typical, water nitrate can be converted to forage nitrate equivalent by dividing by 500. For example, 100 ppm nitrate nitrogen is equal to .2% nitrate ion in the forage. Nitrate in water, however, tends to be more toxic than nitrate in forage, so producers should avoid water higher than 100 ppm nitrate-nitrogen.
Bacterial inoculation to aid in animal adaption. One management option that has recently become available for producers is a commercially available oral paste containing the bacterium propionibacterium P5. This bacterium has the unusual ability to develop a large and stable rumen population and it also has the ability to degrade nitrate to ammonia without the accumulation of nitrite. Research has shown that using this material at least 10 days before nitrate exposure may help prevent acute toxicity. Field experiences with this product have generally been good, but this should be used to provide an extra measure of protection, and should not be used at the exclusion of the many other management practices discussed in this publication.
Sheep, goats and horses. Much less is known about nitrate management in other ruminants as compared to cattle. However, the management considerations and toxicity thresholds discussed in this publication can be applied to sheep and goats. Horses should theoretically be able to tolerate higher nitrate levels than cattle, but very few studies have been conducted to evaluate nitrate toxicity in horses. To be safe, horse owners have been advised to not purchase hay with more than 1% nitrate ion, and to limit dietary level to 0.5% after dilution with other feeds (see "Managing Pastures to Feed Your Horse," NCCES AG-524).
Nitrate toxicity is on the increase in North Carolina. It is important for producers to understand how plants become high in nitrate and how to manage high nitrate forage should they find they have some on hand. This is especially important for producers using nitrate- accumulating forage species, and for those using high levels of nitrogen fertilization on any forage crop. Having a routine testing program, adjusting nitrogen application rates and timing, and applying known animal management techniques will limit the risk of economically devastating problems associated with unknowingly feeding high nitrate forage.